1. Technical Field
The present invention generally relates to reduction of power supply in mobile communication devices.
It finds applications, in particular, while not exclusively, in mobile communication devices such as mobile phones, Smartphones or Personal Digital Assistant (PDAs).
2. Related Art
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Current mobile phones have turned into mobile platforms. Thanks to broadband networks and to the huge increase of the number of software applications, mobile phones are actually inescapable in the daily life. Mobile phones are used for social networking, TV watching, web surfing, as gaming console, for steering and tracking, etc. The limit of their usefulness is now only given by the human imagination to invent new applications.
However, mobile platforms are battery-operated systems and possess finite quantity of embedded energy. The efficiency of all the integrated circuits is challenged in order to ensure the longest autonomy. In addition, very high performance level is required to provide the most comfortable experience to the final consumer and the circuits complexity have increased in an exponential manner. Nevertheless, mobile platforms such as consumer electronic devices have to stand affordable prices with the handiest size.
Audio amplifiers, as an interface with users, are key parts of mobile phones. They are required to achieve very good audio performances, in terms of Signal to Noise Ratio (SNR) and of Total Harmonic Distortion (THD) for example, and to meet High-Fidelity (Hi-Fi) sounds expectations. In this case, AB-class amplifiers are used but they suffer from a poor efficiency, e.g. less than 78%, linked to their voltage supply. One strategy to maximize the efficiency of such amplifiers is to minimize their voltage supply level as much as possible. Amplifiers are then of G or H class. Audio signal transmitted to the speakers is ground centred due to the jack connectors of headphones. Until recently, external high pass filters were used to cut-off the common mode at the output of audio amplifiers. Such filters, made of external capacitors of a few hundred of microfarads, were huge and expensive (8 mm2 each). Then capacitor-less AB-class amplifiers have been designed to get rid of these bulky capacitors. Such amplifiers are supplied by a symmetrical positive and negative voltage which enables to avoid the use of any output common-mode filter.
Thus, audio amplifiers need a symmetrical positive and negative voltage supply to amplify audio signals without common mode.
Formerly, two separated converters were employed to generate one positive and one negative voltage. Using two separated converters requires numerous bulky and expensive external components.
Single Inductor Double Output (SIDO) DC-DC converter providing both a positive and a negative output voltage has been introduced in the document “Single-Inductor Multiple-Output Switching Converters with Bipolar Outputs”, D. Ma, W.-H. Ki, C. Y. Tsui, and P. K. T. Mok, IEEE International Symposium on Circuits and Systems, pp 301-304, vol. 2, 2001.
Such a converter benefits from the high efficiency of inductive DC-DC converters with a minimized number of external components and power switches. An implementation of this Single Inductor Double Output Bipolar converter is described in the document “Dual-Output (Positive and Negative), DC-DC Converter for CCD and LCD”, MAXIM IC, 2003. Shottky diodes are used to simplify the overall control with detrimental impact on efficiency and transient responses.
Single Inductor Multiple Output (SIMO) converters that generate both symmetrical outputs are commonly employed for imaging devices like AMOLED, LCD or CCD. Output voltage levels for such applications are higher than for audio applications. This leads to different power stage architectures such as described in the previous document “Dual-Output (Positive and Negative), DC-DC Converter for CCD and LCD” and in the document Texas Instrument Inc., TPS65136 “POSITIVE AND NEGATIVE OUTPUT DC-DC CONVERTER”, 2008. In this document, the power stage and the conduction scheme do not permit to supply two outputs with non-symmetrical loads like in an audio amplifier. In the previously introduced document “Dual-Output (Positive and Negative), DC-DC Converter for CCD and LCD”, time multiplexing is employed to avoid cross regulation at the cost of an impacted efficiency.
However, it is mandatory for the headphone application to afford high efficiency for low output current too. This is not compatible with a converter working only in CCM.
Thus, there is a need to develop a power supply that is able to afford high efficiency for low output current, while using a minimum of external components.